1. Field of the Invention
The present invention relates to a negative planographic printing plate precursor, in particular to a negative planographic printing plate precursor capable of being directly processed from the digital signals of computers, etc.
2. Description of the Related Art
Printing plates having ink-receiving oleophilic regions and ink-repellent regions (hydrophilic region) that are dampened with water. Various types of photosensitive planographic printing plate precursors (PS precursors) are currently used.
One type of PS precursor now widely used has a photosensitive layer formed on a support such as an aluminium sheet. A PS precursor of this type is exposed imagewise and developed to remove the photosensitive layer in the non-image area, and printing is carried out according to the hydrophilicity of the support surface and the hydrophobicity of the photosensitive layer in the image area. In the non-image area thereof, the PS plate is required not to have any photosensitive layer remaining thereon, but in the image area thereof, the recording layer must be well adhered to the support so it does not easily peel from it. In the non-image area of the PS plate, the hydrophilic support surface is exposed after the recording layer has been removed by development. However, if the exposed support surface is not satisfactorily hydrophilic, ink will adhere to it to cause stains in the prints. Therefore, the support surface of the PS precursor must be highly hydrophilic for preventing the non-image area thereof from being stained.
For the hydrophilic support or the hydrophilic layer of planographic printing plates, aluminium sheets subjected to anodic oxidation to form an oxide film thereon, or oxide film-coated aluminium sheets silicated to further increase their hydrophilicity have generally been used heretofore. Many studies relating to such hydrophilic supports or hydrophilic layers of aluminium are made these days. For example, JP-A No. 7-1853 discloses a support processed with an undercoating agent of polyvinylphosphonic acid; and JP-A 59-101651 discloses a technique of using a polymer containing a sulfonic acid group for the undercoat layer of the photosensitive layer. In addition, a technique of using polyvinylbenzoic acid for the undercoating agent for supports has also been proposed.
On the other hand, flexible PET (polyethylene terephthalate) or cellulose triacetate supports may be used in the place of aluminum metal supports, and various techniques relating to the hydrophilic layer for these have been proposed. For example, JP-A No. 8-292558 discloses a swellable hydrophilic layer comprising a hydrophilic polymer and a hydrophobic polymer; EP 0709228 discloses a PET support having a microporous, hydrophilic crosslinked silicate surface; and JP-A Nos. 8-272087 and 8-507727 disclose a hydrophilic layer containing a hydrophilic polymer and cured with a hydrolyzed tetraalkyl orthosilicate.
These hydrophilic layers are more hydrophilic than conventional ones, and provide plates that produce good prints at the start of printing with no stains. However, the layers are problematic in that they often peel off, and their hydrophilicity lowers by repeated use. Planographic printing plates in which the hydrophilic layer does not peel off from the support, and in which the hydrophilicity of the support surface does not lower even in severer printing conditions and which can produce a large number of good prints with no stains are therefore presently desired. For the reason of a further increase the hydrophilicity of the support surface of planographic printing plates is needed.
Accordingly, the object of the present invention is to provide a negative planographic printing plate precursor that makes possible a negative planographic printing plate capable of forming high-quality images free from the problem of printing stains.